Conventionally, polypropylene films have been widely used in the packaging fields including food packaging, fiber packaging and the like, since they are superior in optical property, mechanical property, packaging applicability and the like. Particularly, composite films having a sealing layer have been widely used as materials for pillow type packages and side-weld sealing bags.
In addition, these films have been produced in large scales in recent years due to high-speed film forming and wide width film forming. What forms a problem in such large-scale production is the thickness irregularity of the film. When a film has thickness irregularity, printability, bag-making property and the like are markedly degraded during film processing steps. With regard to reduction of thickness irregularity, it is comparatively easy for polyester films since they can be easily contacted closely to a roll by electrostatic adhesion method and the like, but the reduction in polyolefin films requires a special production method to achieve smoothness and flatness as shown in patent reference 1, since a melt-extruded film cannot be easily contacted closely to a roll by electrostatic adhesion method, and thickness irregularity tends to develop due to high molten viscosity and wide molecular weight distribution of polyolefin resins, and the relaxation time distribution of molten deformation caused by molecular weight distribution of polyolefin resin and the like patent reference 1: JP-A-7-117124